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Does anastrozole cause weight gain?

Medically reviewed by Carmen Pope, BPharm. Last updated on May 16, 2023.

Official answer

  • Overall, trials have shown no difference in the amount of weight gained between women taking anastrozole and those who were taking tamoxifen or placebo (a pretend pill).
  • However, hormone-reducing therapies may cause weight gain in certain subsets of women because they counteract the effects of estrogen, which helps suppress the activity of an enzyme called LPL that pulls fat into cells.
  • Weight gain may be more likely in women with breast cancer under the age of 60, who smoke, or who have had a mastectomy.
  • Generally, weight gain is commonly reported by breast cancer patients and may also be due to your body shifting into menopause, stress, lack of physical activity due to treatment schedules or tiredness caused by therapy, or sugar cravings.

Anastrozole is a medication that is used to treat hormone-receptor-positive breast cancer in postmenopausal women.

Although many people report weight gain with breast cancer treatment, trials have shown no significant differences between anastrozole and tamoxifen, or anastrozole and placebo (a pretend pill) with regards to weight gain:

  • Over the 12 months of follow-up in the IBIS-II study, women taking anastrozole gained an average of 0.8kg compared to an average of 0.5kg in those taking placebo
  • In the ATAC trial, women taking anastrozole gained on average 1.4kg compared to an average 1.5kg in women taking tamoxifen.

Weight gain tended to occur within the first 12 months of active treatment. None of these differences were statistically significant.

Women were more likely to gain more than 5kg of weight if they were under the age of 60, smoked, or had had a mastectomy.

Many women gain weight with breast cancer treatments, especially if they are also taking steroids such as prednisone. This extra weight can hang around and may appear to increase if you take hormonal therapy (such as anastrozole) and also if your body shifts into menopause because of chemotherapy because women are more likely to gain weight anyway after menopause.

Weight gain may also occur because aromatase inhibitors (such as anastrozole) counteract the effects of estrogen.

An enzyme called lipoprotein lipase (LPL) sits on the surface of cells and pulls fat out of the bloodstream.

  • On a muscle cell it puts fat into the cell where it is used for fuel.
  • On a fat cell, LPL pulls fat into the cell, making it fatter.
  • Estrogen suppresses the activity of LPL on fat cells.
  • Treatments for breast cancer, such as anastrozole, dramatically decrease estrogen levels.
  • With less estrogen in the body LPL can pull fat into fat cells and store it there.

Weight gain may also be due to:

  • Stress or shock caused by your diagnosis
  • Less physical activity due to treatment schedules or feeling fatigued by treatments
  • Financial or relationship stress
  • Yearning for sugary foods caused by treatments
  • Extra fluids
  • Fluid retention caused by steroid treatments.

How to lose weight caused by breast cancer treatments

Excess weight is harder to lose as we get older. Some ways to lose weight include:

  • Count calories
  • Calories should be in the form of nutritious foods, such as fruits and vegetables, not sweets, alcohol, or white bread
  • Avoid or limit alcohol
  • Get moving – try to exercise for at least an hour on four days of the week, more if this is possible
  • Drink 6 to 8 glasses of water a day, especially before meals to make you feel fuller
  • Increase the fiber content of your diet (this will make you feel fuller)
  • Don’t snack between meals
  • Eat slowly and put your food on a smaller plate so it looks like there is more food.

What are the side effects of Anastrozole?

Side effects may start within a day of starting Anastrozole. This is because Anastrozole lowers total body estrogen levels by about 70% within 24 hours of starting the drug, which causes side effects similar to those of menopause, such as hot flashes or difficulty sleeping.

Talk with your doctor if you are having difficulty managing your side effects from Anastrozole.

Common side effects of Anastrozole that affect more than 5% of women and generally start soon after therapy begins to include:

  • Asthenia (weakness or lack of energy)
  • Back, chest, muscle, stomach, or pelvic pain
  • Constipation
  • Cough
  • Diarrhea
  • Headache
  • Hot flashes (flushes)
  • Joint pain
  • Nausea and vomiting
  • Rash
  • Sleeplessness
  • Sore joints
  • Sore throat
  • Vaginal dryness.

Other side effects that generally take weeks or months to develop include:

  • Bone pain
  • Carpal tunnel syndrome (a nerve condition that affects your wrist, causing pain, tingling, or numbness)
  • Depression or low mood
  • Hair thinning
  • High blood pressure
  • High cholesterol
  • Osteoporosis (brittle bones) and fractures
  • Shortness of breath
  • Swelling in the ankles and feet (peripheral edema)
  • Swollen lymph nodes (lymphedema)
  • Weight gain.

The most common reason for discontinuing anastrozole has been hot flashes, although the incidence of this is less than with tamoxifen. Serious side effects that occur in less than 1% of women include:

  • Skin reactions, such as lesions, ulcers or blisters
  • Severe allergic reactions with swelling of the face, lips, tongue or throat
  • Liver toxicity, including liver inflammation and changes in liver function tests.

  • Anastrozole
  • Sestak I, Harvie M, Howell A, Forbes JF, Dowsett M, Cuzick J. Weight change associated with anastrozole and tamoxifen treatment in postmenopausal women with or at high risk of developing breast cancer. Breast Cancer Res Treat. 2012;134(2):727‐734. doi:10.1007/s10549-012-2085-6

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